Apparatus for making polycrystalline articles

ABSTRACT

A polycrystalline article is densified to provide either a nonporous body or a body with controlled interconnected porosity. A mixture of fine powders of the polycrystalline material and a sintering aid is compacted and outgassed under reduced pressure. The outgassed compact is then subjected to a permeation anneal step in which it is heated in a closed chamber to a temperature sufficient to form a liquid of the sintering aid, but under pressure conditions which inhibit evaporation of the sintering aid. The sintering aid can then be leached out to provide a densified article having interconnected porosity. Alternatively, the sintering aid can be leached out at elevated temperature, further densifying the compact to form a substantially nonporous body. Alternatively, the sintering aid can be removed by subjecting the densified article to an evaporation anneal step in which the article is heated to evaporate the sintering aid, further densifying the compact to form a substantially nonporous article. Apparatus is provided containing interconnected sections to accomplish the foregoing process. When applied to magnesium fluoride, a transparent polycrystalline body is obtained as a new article of manufacture which is substantially uniformly transparent to infrared radiation throughout the entire range of 0.7-6 microns.

This is a division of application Ser. No. 936,540, filed Aug. 23, 1978,abandoned which, in turn, is a division of application Ser. No. 827,279,filed Aug. 24, 1977 now U.S. Pat. No. 4,146,379.

FIELD OF THE INVENTION

The fields of art to which the invention pertains include the fields ofarticle shaping or treating, particularly as applied to opticalarticles, and the field of sintering.

BACKGROUND AND SUMMARY OF THE INVENTION

Many articles of manufacture are termed by densification of finepowders, such as radomes, laser windows, battery electrolytes, cuttingtools, penetrators, electrooptical devices, fuel elements, and ceramicturbine blades. It is known that volatile additives can be employed topromote the densification by sintering of fine powders. For example,Atlas in U.S. Pat. No. 2,823,134 teaches the use of lithium compounds inthe sintering of cold-pressed magnesium oxide powder. During heating,these compounds are believed to form a liquid thereby increasing therate of sintering of the powder while at higher temperatures they arebelieved to evaporate producing a relatively pure, high density article.Although an object of the Atlas invention was to provide an improvedmethod for densifying magnesium oxide, Atlas was not able to obtainnonporous articles by his method. Rice et al in U.S. Pat. No. 3,301,781teach the use of lithium halide salts in the sintering of cold-pressedmagnesium fluoride. During heating, these compounds are also thought toform a liquid. Rice also was not able to obtain pore-free articles. Onedifficulty in both the Altas and Rice et al processes is entrapment ofinsoluble gases by the volatile additive liquid when it melts and flowsthroughout the compacted powder. Another difficulty is the evaporationof the additive from the powder before all the benefits are obtained.

It is also known that atmosphere control can be employed to enhance theeffect of volatile additives. Snow Reports ["Fabrication of TransparentElectrooptic PLZT Ceramics by Atmosphere Sintering", J. of Amer. Ceram.Soc., 56(2) pp. 91-96, 1973; "Improvements in Atmosphere Sintering ofTransparent PLZT Ceramics", J. of Amer. Ceram. Soc., 56(9) pp. 479-480,1973] that large nonporous plates of lanthanum-modified leadzirconate-titanate (PLZT) can be produced by sintering powders withexcess PbO present as a volatile additive. To produce nonporous plates,the powders are first cold-pressed to form slugs, then annealed inoxygen in a platinum crucible and finally annealed in an aluminacrucible containing PbZrO₃ powder to provide a PbO-rich atmosphere. InSnow's process, oxygen, which is soluble in the liquid, is entrapped bythe liquid as it flows throughout the body. The oxygen can escape fromthe pores by diffusion, allowing the body to shrink. Also, containmentof the cold-pressed slugs in the platinum crucible decreases the rate ofloss of the PbO-rich liquid by evaporation and thus allows the fullbenefits of the liquid to be obtained. A goal in Snow's work was toprepare transparent and thus pore-free plates of PLZT. Although thisgoal was apparently realized, there are several difficulties in applyingSnow's process to a broader range of materials. One difficulty is thelimited opportunity for outgassing the power inherent in the process.Another difficulty is the use of the soluble gas during the initialannealing treament. It may not be possible to obtain a suitable gas inmany systems. Still another difficulty is the continuous loss of thevolatile additive during the Snow process. Although containment in thecrucible reduces this rate of loss sufficiently in the case of thePbO-rich liquid, it might not reduce it sufficiently in the case of morevolatile additives for the full benefits of the liquid to be obtained.

In addition, it is known that volatile additives can be employed topromote the densification of time powders by hot-pressing. Carnall, Jr.,in U.S. Pat. No. 3,476,690 teaches the use of LiF as an additive in thehot-pressing of MgO to form pore-free, optically useful elementstransparent to both visible and infrared radiation. Similarly,transparent Y₂ O₃ and MgAl₂ O₄ have been obtained by hot pressing withLiF. The use of volatile additives in hot pressing has been proposed inother systems including the densification of CaO, Al₂ O₃, and BaTiO₃.Also, NaF has been proposed as a volatile additive in the hot-pressingof MgO. Although enhanced densification has been observed with thesesystems, it is not clear that one could obtain nonporous bodies in allcases. One difficulty is the limited opportunity for outgassing thepowder due to its containment in the hot-pressing die. Also, hotpressing is generally considered to be a more expensive process formaking articles than is cold-pressing and sintering.

In addition to the foregoing deficiencies, it has not been an objectivein the previous work to make articles with inter-connected channels in acontrolled manner or to make articles with controlled amounts ofinter-connected porosity, or to make articles with other than simpleshapes. Such articles would be highly advantageous, for example as highsurface area cylindrical supports for platinum or palladium catalysts,or as shaped ceramic filters for use in such apparatus as soxhletextractors.

The present invention overcomes the foregoing drawbacks and provides aninexpensive process and apparatus for densifying a polycrystallinearticle formed from an inorganic compound in fine powder form. Theprocess is capable of producing substantially nonporous articles from awide variety of inorganic compounds or it can be operated to produce anarticle having a controlled amount of interconnected porosity. Theapparatus provided herein enables the process to be conducted in arather simple manner with sufficient flexibility to permit thefabrication of shaped articles having nonporous or controlled porositycharacteristics.

Specifically, the process provided herein applies to the densificationof a polycrystalline article formed from an inorganic compound in finepowder form in which the powder is mixed with a sintering aid, themixture is compacted to a predetermined shape and then sintered. Theprocess provides an improvement according to which prior to thesintering step, the compact is heated and subjected to temperature andpressure conditions to outgas the compact but the conditions areinsufficient to form a liquid of the sintering aid. The outgassedcompact is then subjected to a permeation anneal step in which it isfurther heated in a closed chamber under temperature and pressureconditions which serve to inhibit evaporation of the sintering aid, fora time sufficient for the liquid to permeate and substantially densifythe compact. Thereafter, the sintering aid is removed from the densifiedcompact.

In one embodiment the sintering aid is removed by subjecting thepermeated compact to an evaporation anneal step by heating underpressure conditions which permit the sintering aid to evaporate wherebyto obtain a substantially nonporous article. In another embodiment, thesintering aid is removed by leaching it from the densified compact withan appropriate liquid solvent at a suitable temperature whereby toobtain an article having a substantial quantity of interconnected pores.The basic features of the liquid solvent include both limited solubilityand wettability for the polycrystalline material and at least moderatesolubility for the sintering aid.

After the permeation anneal step, but prior to removing the sinteringaid, the densified compact can be forged to a desired shape, andproviding that temperature and pressure conditions do not permitevaporation of the sintering aid, an article having that shape can beproduced with controlled interconnected porosity by thereafter leachingout the sintering aid. Alternatively, the compact can be subjected toconditions which evaporate or solvent-extract the sintering aid tofurther densify the compact, producing a substantially nonporous articleof that shape.

The specific technique used in the present invention to accomplish theforegoing involves conducting the permeation anneal in a closed chamberin which there has been placed a quantity of "atmosphere" material,having a volatility at least as high as the sintering aid to provide anoverpressure. The atmosphere material may be of the same composition asthe sintering aid in which case the apparatus is subjected totemperature differentials to heat the atmosphere material at a highertemperature than the compacted sintering aid during the permeationanneal thereby inhibiting evaporation of the sintering aid. During theevaporation anneal step, the compact is heated at a higher temperaturethan is the atmosphere powder, or the compact is heated exposed to theambient atmosphere, to evaporate the sintering aid causing furtherdensification to a nonporous state.

The apparatus used in conducting the foregoing process comprises avessel having a cavity with means in the cavity for defining a reservoirsection containing atmosphere material and a specimen section incommunication with each other, and seal means for containing a liquidseal for the cavity. The seal means can comprise a channel formedthrough the apparatus from the outside to the reservoir section, openinginto that section adjacent the bottom thereof. In one embodiment, thecavity sections are defined in horizontally spaced relation and a wallis provided between the sections defining a passage thereacross. Tightfitting caps close the sections and are removable for purposes ofloading the apparatus. The channel is formed through the cap closing thereservoir section. In another embodiment, the specimen and reservoirsections are defined as top and bottom sections, respectively, invertically spaced relation between a top wall and a bottom wall and apedestal is provided to support the specimen spaced from the bottom wallof a cavity. In this embodiment, the top wall can be formed with a capresting on the top edge of the apparatus and which is removable from theapparatus. The top edge is formed with a channel constituting the sealmeans and the lid is formed with a lip fitting in the channel.

The process of this invention is capable of forming transparent articlesfrom such compacted compounds as magnesium oxide or magnesium fluoride.Particularly with respect to magnesium fluoride, a novel article ofmanufacture can be produced. Polycrystalline magnesium fluoride windowsare transparent to infrared but those currently available are notuniformly transparent throughout the entire region of the infraredspectra, exhibiting one or more undesirable absorption bands at 2.7,3.0, 5.0, 6.2 or 6.7 microns, that are detrimental to its use aswindows, for example for chemical lasers. By practicing the presentinvention, a body of magnesium fluoride can be produced having aporosity of less than 0.1 volume percent, having a grain size of lessthan 10 microns and which is substantially uniformly transparent toinfrared radiation throughout the entire range of 0.7-8 microns.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of apparatus for use with the presentinvention including in a specimen section a compact to be densified and,in a reservoir section, a liquid forming seal material;

FIG. 2 is a view similar to that of FIG. 1 but in which the sealmaterial has been liquefied;

FIGS. 3A-3F are schematic drawings illustrating the principal steps ofthe process using the embodiment of FIG. 1;

FIG. 4 is a cross-sectional view of alternative apparatus used in thepresent invention;

FIG. 5 is cross-sectional view of the apparatus of FIG. 4 taken on line5--5 thereof, showing a plan view of a spacer component of saidapparatus;

FIGS. 6A-6F are schematic drawings illustrating the principal steps ofthe process using the embodiment of FIG. 4;

FIGS. 7A and 7B are schematic illustrations of the solid-vaporinterfaces and liquid-solid interfaces, respectively, of a three grainjunction of the compacted mixture densified by the present invention;

FIG. 8 is a photomicrograph of compact formed of 80 wt. % LiF, annealedin accordance with the present invention showing spherical MgO particlessurrounded by the LiF additive;

FIG. 9 shows a compact similar to that shown in FIG. 8, but after LiFhas been leached out, and showing interconnected porosity between theMgO grains; and

FIG. 10 is a photomicrograph of the fracture surface of a nonporous MgOcompact from which LiF has been removed by evaporation.

DETAILED DESCRIPTION

As required, details of illustrative embodiments of the invention aredisclosed. However, it is to be understood that these details merelyexemplify the invention which may take forms different from the specificillustrative embodiments. Therefore, specific structural and functionaldetails are not necessarily to be interpreted as limiting, but simply asa basis for the claims. Additionally, specific examples will be givenwith respect to the specific inorganic compounds. One of the advantagesof the invention is that it has application to a broad range ofdifferent types of compounds.

Referring now to FIG. 1, there is shown apparatus in accordance with anembodiment of the present invention. The apparatus consists of acylindrical graphite vessel 10 having a cavity divided into a specimensection 12 and a reservoir section 14 separated by a wall 16. An opening18 through the wall 16 connects the two sections 12 and 14. The vessel10 is closed at its opposite ends by a pair of tight press fitted caps20 and 22 (which, alternatively, could be thread-fitted). One of thecaps 22 is formed with a passageway 24 defining an oblique channel fromthe outside of the vessel opening into the reservoir section 14 adjacentthe bottom thereof. Liquid forming seal material 26 is placed in thatsection while the compact 28 to be densified is placed in the specimensection 12. The compact 28 is formed by cold-pressing a mixture of thebase material and a sintering aid. The liquid forming seal material 26is advantageously in the form of chunks of the material which will servenot only as a liquid seal material but also as a means for providingoverpressure of the sintering aid during liquid phase sintering of thecompact 28 to inhibit evaporation of the sintering aid from the compact28. The seal material 26 can have the same composition as the sinteringaid. Referring to FIG. 2, as will be described in more detail hereafter,when the vessel 10 is heated above the melting point of the sealmaterial 26, it forms a liquid which plugs the channel 24 providing aneffective liquid seal for the vessel 10.

The process of the present invention is conducted in essentially foursteps:

(a) Fine powders of an inorganic compound to be densified are mixed witha sintering aid. The mixture is compacted to a simple shape to providethe initial form of the article 28.

(b) The compact is outgassed by heating under reduced pressure but belowthat temperature which would form a liquid of the sintering aid.

(c) A permeation anneal step is conducted in which the outgassed compactis heated under pressure conditions sufficient to inhibit evaporation ofthe sintering aid, to densify the compact.

(d) The sintering aid is removed from the densified compact. Removal canbe accomplished by a chemical leach (i.e., liquid solvent extraction)step to produce an article with interconnected porosity or the compactcan be subjected to an evaporation anneal step in which it is heatedunder temperature and pressure conditions sufficient to evaporate thesintering aid, thereby further densifying the compact to produce anonporous article. Liquid solvent extraction can also be used to removethe sintering aid and yield a nonporous body. In this case, it isnecessary that the liquid solvent have at least a moderate solubilityfor the sintering aid but a limited wettability for the polycrystallinematerial as well as a limited solubility for it.

Referring to FIGS. 3A-3F, the foregoing outgassing, permeation annealand evaporation anneal steps are illustrated using the apparatus ofFIG. 1. In FIGS. 3A, 3C and 3E, the temperature distribution of theapparatus of FIGS. 3B, 3D and 3F, respectively, are shown wherein T₁ isthe temperature of the specimen section 12 containing the compact 28 tobe densified and T₂ is the temperature of the reservoir section 14containing the seal material 26. The designation T_(M) represents thetemperature at which the sintering aid portion of the compact 28liquifies. The letter designation T_(S) represents the sinteringtemperature. As indicated above, the seal material 26 can be differentfrom the sintering aid, but conveniently, it is formed of the samecomposition so that overpressure conditions and underpressure conditionsare controlled entirely by the relative temperatures T₁ and T₂.

In each of FIGS. 3B, 3D, and 3F, the vessel 10 is shown placed in ahorizontal, quartz tube 30 which is placed in a horizontalresistance-heated furnace having temperature and thermal gradientsadjusted appropriately to yield the temperature distribution atdifferent places along its length shown immediately thereabove in therespective FIGS. 3A, 3C and 3E.

Prior to insertion of the vessel 10 in the furnace, the compact 28 isformed by mixing together desired base material, such as magnesiumoxide, with a sintering aid, such as lithium fluoride in an appropriateamount. The mixture is cold pressed to form a specimen 28 in the form ofa slug as illustrated. A quantity of the sintering aid 26 is placed asseal material in the adjacent vessel section 14. Referring specificallyto FIGS. 3A, and 3B, the quartz tube is evacuated as indicated by thearrow 32 and heated to a temperature which is below the point at whichthe sintering aid liquifies, and is maintained at that temperature untilabsorbed water is driven off from the compact 28. Such outgassing takesplace through the channel 24. At this point, both sections of the vessel10 are heated to the same temperature. Depending on the material,reactions such as binder burn-out, dehydration or calcination as well asoutgassing may be carried out by holding the compacted mixture 28 at anelevated temperature for a sufficient period of time. Furthermore, byadding a reactive gas to the furnace chamber, such as dry hydrogen orother reducing gaseous mixture, surface deoxidation can be carried outwith some materials. With semiconducting materials, doping can also becarried out by a reactive gas technique. The furnace is evacuatedfollowing such reaction but prior to the next step.

Referring specifically to FIGS. 3C and 3D, the temperature of thefurnace chamber is raised above the point at which the sintering aidliquifies serving also to melt the seal material which rises in thechannel 24 to form a liquid seal for the vessel. This may beaccomplished by moving the quartz tube 30 into a part of the furnacewhere the temperature distribution is as shown in FIG. 3C. Thetemperature distribution is such that the reservoir section 14 is at ahigher temperature than the specimen section 12, both sections being ata temperature above the point at which the sintering aid becomes liquid.As a result of the liquid seal, an overpressure of the sintering aidinside the vessel can be maintained preventing evaporation of thesintering aid from the compact 28. Thus, the sintering aid in thecompact 28 melts and in response to capillary forces infiltrates thecompact. However, it cannot evaporate from the body because thereservoir 26 of sintering aid is hotter than the compact 28. At thisstage, a permeation of the compact by the liquid takes place in whichthe pore volume decreases with time due to a liquid phase sinteringprocesses.

After the permeation anneal step, the evaporation anneal step isconducted as illustrated in FIGS. 3E and 3F. The temperature of thereservoir section 14 and specimen section 12 are both still above thepoint at which the sintering aid melts but the specimen section 12 isnow maintained at a higher temperature than is the reservoir section 14.Under these circumstances, the sintering aid is permitted to evaporatefrom the compact 28 at a rate controlled by the temperature differencebetween the specimen section and the reservoir section (T₁ -T₂) furtherdensifying the compact 28 to form a substantially nonporous article. Theevaporation anneal step could also be conducted in an ambientatmosphere.

Referring now to FIGS. 4 and 5, an alternative form of apparatus isshown. A cylindrical graphite vessel 32 is provided in which a specimensection 34 and reservoir section 36 are defined as top and bottomsections, respectively, in vertically spaced relation between a top wall38 and bottom wall 40. The top wall is in the form of a cap having adependent lip 42 which loosely fits and rests in a channel 44 formedthrough the top edge 46 of the vessel 32. Liquid forming seal material48 is placed in the channel so that when the lid is supported in thechannel, and the seal material is melted, a liquid seal is formed. Slugsof compact 50 are supported in the specimen section between graphitespacers 52 and the assembly is supported on a graphite pedestal 54 whichitself rests on the bottom wall 40 of the vessel 32. A quantity ofatmosphere material 56 which can be of the same type as referred to withrespect to FIG. 1, is placed in the bottom section 36. As shown, holes57 are formed in the graphite spacers 52 and slots 59 are cut on bothsurfaces to assist the reservoir section 36 in establishing a partialpressure of the atmosphere material 56 throughout the vessel and aroundthe compact.

It will be appreciated that in place of the top edge channel 44containing the liquid forming seal material 48, one could provide anoblique channel into the reservoir section 36 such as illustrated in theapparatus of FIG. 1.

Referring now to FIGS. 6A-6F, various processing steps utilizing theapparatus of FIG. 4 are illustrated. In FIGS. 6A, 6C and 6E, thetemperature distribution of the apparatus of FIGS. 6B, 6D and 6F,respectively, are shown where T₁ is the temperature of the specimensection 34 and T₂ is the temperature of the reservoir section 36. Theletter designations T_(M) and T_(S) are as given above with respect toFIGS. 3A-3F.

As with the sequence of FIGS. 3A-3F, initially at FIGS. 6A and 6B, theapparatus is placed in a furnace 58 and outgassed at a temperature whichis below that required to liquefy the sintering aid-seal. The processcan be conducted with the lid in place as it will rest upon chunks ofthe sintering aid permitting flow therethrough. Alternatively, the lidcan be lifted during outgassing.

During the permeation anneal step, as illustrated in FIGS. 6C and 6D,the temperature of the vessel is raised above the point at which thesintering aid liquefies but with the compact 50 maintained at atemperature somewhat less than the temperature at which the reservoir ismaintained so as to obtain the desired overpressure. After thepermeation anneal step, an evaporation anneal step can be conducted bylowering the vessel 32 in the furnace 58 so that the reservoir 56 is ata temperature below that of the compact 50, but still above the point atwhich the sintering aid liquefies so that the sintering aid canevaporate from the compact 50, forming a nonporous article. Theevaporation anneal step could also be conducted in ambient atmosphere.

It will be apparent from the foregoing discussion that the liquid sealforming material, atmosphere powder and sintering aid can all be ofdifferent composition but also can all be of the same composition.

In place of the evaporation anneal step, one can simply chemically leachthe sintering aid from the densified compact to provide a body having adesired degree of interconnected porosity. Leaching is accomplished witha liquid that reacts with the sintering aid or dissolves it to a muchgreater extent than the polycrystalline material. With lithium salts, asuitable leaching agent is simply water.

Both the permeation anneal step and the evaporation anneal step havebeen subject to some extensive theoretical considerations. When thecompact is heated above the melting point of the sintering aid, a liquidforms and in response to capillary action permeates the compact. Poresmay be initially present, because the liquid is not normally sufficientin volume to fill all the space among the particles. The pore volumedecreases with time due to liquid phase sintering proceses involvingparticle rearrangement aided by viscous flow and by dissolution atcontact points and reprecipitation elsewhere in the compact. At theconclusion of the permeation anneal step, all the pore volume iseliminated and the compact consists only of the polycrystalline materialand the liquid sintering aid.

During the evaporation anneal step, further densification takes place.As the liquid evaporates from the compact, the microstructure of thecompact changes from a three dimensional skeleton of spheroidalparticles joined by necks and surrounded by liquid to an assembly ofangular grains with a continuous network of liquid filled channels atthree grain junctions. These changes, which cause the compact todensify, occur in a very short period of time indicating high transportrates. For a wide range of annealing conditions, a pore-free,polycrystalline compact can be obtained. While it is not desired to berestricted to any particular mechanism of operation, it can be theorizedthat a negative hydrostatic pressure is present in the liquid in thecompact. The magnitude of this pressure is determined by the curvatureof the liquid-vapor interface where the liquid emerges at the surface ofthe compact. Because of this pressure, compressive stresses develop atthe interfaces between particles, i.e., at grain boundaries. Atomsmigrate down the gradient in chemical potential resulting from thecompressive stresses and deposit at the solid-liquid interfaces. Thus,the grains become angular, the grain centers move closer together andthe interstices shrink to form a three dimensional network of channelslying along the three grain junctions. Liquid flows from the interior ofthe compact to the outer surface where it evaporates. If the rate ofliquid flow becomes to great, the curvature of the liquid-vaporinterface decreases resulting in a decrease in the negative hydrostaticpressure and a decrease in the shrinkage rate of the interstices. It isbelieved that the rate of shrinkage of the interstices is normallysufficient to supply enough liquid flow to the surface so that theliquid-vapor interface does not retreat into the compact.

A remarkable aspect of the changes that occur during the evaporationanneal is the stability of the liquid containing channels at the threegrain junctions. In contrast to the behavior of cylindrical pores, whichnormally pinch off to form a series of spherical pores, the liquidfilled channels appear to be stable, providing a continuous path for thetransport of liquid from the interior of the compact to the outersurface during the evaporation anneal. It is believed that thesechannels are stable for the reasons illustrated in FIGS. 7A and 7B. Theshape of a phase at a three grain junction is known to depend on therelative values of the grain boundary energy and the interphaseinterface energy. The dihedral angle θ must satisfy the equation cos(θ/2)=γ/2γ' where γ is the grain boundary energy and γ' is the energy ofthe interphase interface. If the phase at the three grain junction is acylindrical pore, they γ' corresponds to the energy of a solid-vaporinterface (γ_(sv)). For most materials γ<√3γ_(sv) so that thecylindrical pore is shaped as shown in FIGS. 7A. It is well known thatgrain boundary phases of this shape are unstable and pinch off to form aseries of spheroids lying along the three grain junction [C. S. Smith,"Grains, Phases and Interfaces: An Interpretation of Microstructure,"Trans. AIME 175, pp. 15-51, 1948]. If the phase at the three grainjunction is a liquid filled channel, then γ' corresponds to the energyof a solid-liquid interface (γ_(ls)). For a wetting liquid this isnormally much less than the energy of a solid-vapor interface so thatthe condition γ>√3γ_(ls) ' is likely to be satisfied and theliquid-filled channel is shaped as shown in FIG. 7B. It is well knownthat grain boundary phases of this shape are stable as indicated by theC. S. Smith article above cited.

It will be appreciated that specific temperature, pressure, particlesize and concentration parameters cannot be set forth definitively butdepend on the specific compositions and compounds employed. Theprinciples of this invention apply to any of the inorganic compoundswhich have been successfully densified in the past and can be applied toother compounds which would be subject to densification by sintering.The identities of such material and the various parameters associatedwith their selection are either well known from prior work by others orcan be readily determined by simple trial and error guided by theexamples which follow below. Reference can be made to the variouspatents and publications referred to above for background information,the disclosures thereof being expressly incorporated by referenceherein. Additionally, the disclosures of the following patents areincorporated expressly by reference herein: U.S. Pat. No. 3,589,880,Canadian Pat. No. 646,436, U.S. Pat. No. 3,131,025, Canadian Pat. No.701,645, Canadian Pat. No. 688,568, U.S. Pat. No. 3,236,595, CanadianPat. No. 731,706, U.S. Pat. No. 3,475,116, U.S. Pat. No. 3,206,279,Canadian Pat. No. 723,556, Canadian Pat. No. 706,300, Canadian Pat. No.727,530, U.S. Pat. No. 3,459,503 and U.S. patent application Ser. No.517,965 (the latter assigned to Eastman Kodak Company, and reportedlynow abandoned). Additionally, the following articles can be referred to,the disclosures of each of which being expressly incorporated byreference herein: R. A. Lefever and John Matsko, "Transparent YttriumOxide Ceramics," Mater. Res. Bull. 2 (9), 665-669 (1967); R. W. Rice,"CaO: I. Fabrication and Characterization," J. Am. Ceram. Soc. 52 (8),420-427 (1969); R. W. Rice, "CaO: II. Properties," J. Am. Ceram. Soc. 52(8), 428-436 (1969); W. M. Rhodes, P. L. Berneburg and J. L. Niesse,"Development of Transparent Spinel (MgA)₂ O₄)," Tech. Rpt. DAAG46-69-C-D113 October 1970; R. W. Rice, "Fabrication and Characterizationof Hot Pressed Al₂ O₃," Tech. Rpt. NNG 7111, 1970; B. S. Walker, R. W.Rice and J. R. Spann, "Influence of Additives on the Densification andProperties of Sintered and Pressure Sintered BaTiO₃," presented at the72nd Annual Meeting of the American Ceramic Society, May 6, 19709,Philadelphia, Pa. (Basic Science Division No. 36-L-70) [for abstract seeAm. Ceram. Soc. Bull. 49 (4), 420 (1970)]. Among the materials which arecandidates for the present process are magnesium oxide, magnesiumfluoride, calcium oxide, calcium fluoride, zinc selenide, cadmiumtelluride, gallium arsenide, lanthanum fluoride, cadmium sulfide, zincoxide, strontium fluoride, barium fluoride, titanium dioxide, cadmiumselenide, cadmium iodide, magnesia-alumina spinel, yttrium oxide,alumina, barium titanate, tungsten carbide, silicon carbide, siliconnitride, and aluminum nitride.

The nature of sintering bias is well known to the art. Generally, itshould be a liquid at the sintering temperatures and have a smallsolubility in the material being sintered. On the other hand, thesintered material should have some solubility up to a moderatesolubility in the sintering aid at the sintering temperature. When it isdesired to produce a nonporous article, the sintering aid should bevolatile at reasonable temperatures. The sintering aid should wet theparticle surfaces of the material to be sintered and may also includethe ability to flux undesirable surface materials such as oxides,chlorides, sulfides, or organic materials on the particles to besintered. Of course, chemical compatibility of the sintering aid and theparticles is assumed. In general, when the sintering aid is in liquidform, it should promote rearrangement of the sintered particlesconsistent with desired final density and porosity. For many of thespecific materials above referred to, lithium compounds can be used assintering aids. Generally, such lithium compounds are used which do notcontain any other metallic atoms or ions and candidates include lithiumfluoride, lithium chloride, lithium bromide, lithium iodide, lithiumsulfate, lithium carbonate, lithium nitrate, and the like, as well asmixtures of such compounds. Other candidates include the pentoxidecompounds of vanadium, phosphorous and arsenic.

The amounts of sintering aid used in the practice of the instantinvention are preferably computed on the basis of the "equivalent"percent of the sintering aid. Such equivalent percent is computed as themol percent of the sintering aid in the mixture multiplied by the numberof metallic atoms in the molecule. For example, in a mixture of 100parts MgO and 1 part lithium fluoride, mols are as follows (assumingcompletely pure compounds):

    MgO: 100/MgO mol. wt=100/40.3=2.48 mols

    LiF: 1/LiF mol. wt.=1/25.9=0.0366 mol

The total mols are 2.48+0.0386=2.5186; and the mol percent of lithiumfluoride is

    (0.0386/2.5186)×100=1.53 mol% LiF

Since lithium fluoride has only one lithium atom in the molecule, theequivalent percent is the same as the mol percent; but, if in a mixtureof magnesia (MgO) and lithium sulfate, the Li₂ SO₄ mol percent is 0.94,for example, then the equivalent percent of lithium sulfate is twice themol percent or 1.88 equivalent percent because there are two lithiumatoms in the molecular formula for lithium sulfate. The amount ofsintering aid used may vary from little more than a trace, for example,as low as 0.01 equivalent percent, to whatever is found to be a maximumpractical amount, which can be as high as 30 equivalent percent.Preferably, the amount used is from about 0.05 to about 5 equivalentpercent.

The compact which is to be densified is formed of fine powders of theinorganic compound, preferably less than 25 microns in diameter.Particularly when it is desired to form optical elements which aresubstantially transparent, very fine particles sizes should be used,generally less than 10 microns and preferably less than 1 micron indiameter. In this regard, high purity magnesium fluoride (MgF₂) powdercan be compacted with lithium fluoride as a sintering aid and Jensifiedusing the outgassing, permeation annealing and evaporation annealingsteps, as previously described, to obtain a completely dense articlewith a small grain size (1-10 microns) and which can be mechanically orchemically polished to provide a very smooth surface and a transparencywhich is substantially uniform over the entire infrared range to providea polycrystalline infrared transparent optical element usable as a laserwindow, as a lens, as a prism, or the like.

The following examples, in which parts are by weight, will furtherillustrate the invention.

EXAMPLE 1

Lithium fluoride was purified by zone melting and the solution ingotsections were crushed and ball milled to obtain a fine powder. Thepowder was then sieved to obtain particles of less than 15 microns indiameter. Eighty parts of reagent grade MgO powder having a averageparticle size of about 0.0145 micron was mixed with about 20 parts ofthe lithium fluoride powder and the mixture was blended ultrasonicallyin a medium of isopropyl alcohol for 45 minutes. The alcohol was thenevaporated to obtain the powder. Discs were formed by cold-pressing thepowder under a pressure of 30,500 pounds per square inch provided by adouble action tool steel die. Compacts were thus obtained which were 1mm. thick and 12.7 mm. in diameter.

A specimen compact was placed in the specimen section of a vessel suchas is illustrated in FIG. 1. Sufficient chunks of lithium fluoride wereplaced in the reservoir section of the vessel so as to almost fill thevessel. The vessel was then placed in a quartz tube which could be movedinto or out of preheating and annealing furnaces arranged in tandem.

Initially, the quartz tube was evacuated and heated to 500° C. andmaintained at that temperature for a period of 12 hours permittinggaseous impurities to be released from the powder and effuse from thevessel. With appropriate thermocouples attached, the quartz tube waspushed into the hot zone of the preheating furnace which was maintainedat about 1000° C. The reaction vessel was rapidly heated to a permeationanneal temperature of 860° C. and in about 150 seconds. The quartz tubewas then pushed into the sintering furnace and was maintained for about90 minutes at about 860° C. with the reservoir section of the vessel ata temperature of about 2° C. higher than the specimen section of thevessel. The melting point of lithium fluoride is 842° C., so that uponheating in the sintering furnace, the lithium fluoride pieces in thereservoir section of the vessel melted and sealed the vessel by risingin the oblique channel provides for that purpose. Referring to FIG. 8, aphotomicrograph is shown, at 20,000X magnification, of the fracturesurface of the compact of MgO and LiF prepared as above. The grains arespheroidal in shape and are coated with a film.

EXAMPLE 2

The procedure of Example 1 is followed but the permeated compact wassubject to evaporation annealing by heating at a rate of 50° C./hour to1300° C. and holding at that temperature for 2 hours. After evaporationannealing, the compact showed grains that were angular rather thanhaving a spherical shape similar to that shown in FIG. 10. The fracturepath was transgranular as well as intergranular. There was no indicationof grain boundary phases or porosity at three grain junctions.

EXAMPLE 3

The procedure of Example 1 is followed except that in place ofevaporation of the lithium fluoride, the compact was placed in a beakerof water at room temperature for a period of 5 days to leach out thelithium fluoride. The compact was then washed several times in the waterand dried. Referring to FIG. 9, a photomicrograph is shown at the samemagnification and of the same compact as in Example 1 but after theforegoing leaching step. The fracture surface in FIG. 8 shows a filmcoating the magnesium oxide particles and that considerable growth ofmagnesium oxide particles occurred during the isothermal annealingperiod. The microstructure of the leached fracture surface in FIG. 9reveals that the magnesium oxide particles are spheroids about 0.1micrometer in diameter that have sintered together forming necks. Thecompact has a high degree of interconnected porosity.

EXAMPLE 4

Ninety five parts of magnesium oxide powder were intimately mixed with 5parts of lithium fluoride powder, each having the purity and particlesize characteristics referred to in Example 1. The mixture was thencompacted in tool steel dies at pressures of 30,500 psi. The compact wasplaced on a graphite pedestal in a reaction vessel as described withrespect to FIG. 4. A quantity of lithium fluoride was placed on thefloor of the reaction vessel sufficient to well cover the floor butspaced below the lowermost graphite spacer. Additionally, chunks oflithium fluoride were placed in the channel along the top edge of thereaction vessel and the cap was placed on the vessel with the lip of thecap in the channel supported by the lithium fluoride.

Initially, the vessel was heated to 379° C. and kept there for one halfhour in order to drive out any condensed moisture present. The vesselwas then heated to 800° C. and kept there for one hour in order toremove any occluded gases from the compact. To carry out liquid phasesintering, the temperature of the compact was raised to 925° C. Duringthe heat-up, lithium fluoride present in the channel along the top edgeof the vessel melted and the cap, because of its weight, lowered intothe liquid, thereby sealing the vessel. The temperature of the lithiumfluoride reservoir at the bottom of the vessel was about 20° C. higherthan the temperature of the compact throughout this stage. The receiverthus provided an overpressure of lithium fluoride sufficient to preventevaporation of the lithium fluoride from the compact. The compact wasmaintained at this temperature for about 25 minutes permittingcompletion of the liquid phase sintering of the compact and leading tothe complete removal of the pores. The compact at the end of this stepconsisted of solid magnesium oxide and liquid lithium fluoride only. Thevessel was then cooled and the compact was taken out. The compacts werethen placed in a globar furnace on an alumina pedestal and heated slowlyto 1300° C. and kept there for three hours to produce a substantiallynonporous highly densified article.

EXAMPLE 5

A compact can be prepared as in Example 1 but wherein of magnesiumfluoride (MgF₂) is mixed with lithium fluoride powder. The compact canbe placed in the specimen section of a vessel such as shown in FIG. 1and a quantity of lithium fluoride placed in the reservoir section ofthe vessel. The vessel can then be placed in a quartz tube as shown inFIG. 3. The vessel can be evacuated and the vessel and tube placed in afurnace chamber heated to outgas the compact and maintained at thattemperature until absorbed water is driven off from the compact. Thetemperature of the furnace chamber can then be raised to above themelting point of lithium fluoride with the reservoir section of thevessel maintained at a temperature a few degrees higher than thetemperature of the specimen section. Lithium fluorine in the reservoirmelts and plug the oblique channel in the vessel so that a lithiumfluoride atmosphere inside the vessel is maintained. After permeationanneal has been completed, the vessel can be moved to a new location inthe furnace chamber where the reservoir is at a temperature lower thanthe compact. Evaporation can be continued until all of the lithiumfluorine is removed from the compact.

By conducting the foregoing procedure, one can obtain a body ofmagnesium fluoride which is completely dense and transparent. The bodywill have uniformly high transmission to infrared radiation up to about8 microns will not have the absorption bonus normally associated withpolycrystalline magnesium fluoride bodies manufactured by such methodsas hot pressing.

We claim:
 1. Apparatus for densifying at least one compact comprisingpowder to be densified mixed with a sintering aid therefor, underchanging pressure conditions in response to a change in temperatureapplied to said apparatus, comprising:a vessel having a cavity; meansfor defining in said cavity a specimen section for containing a compactto be densified and a reservoir section containing atmosphere materialfor providing overpressure during sintering to aid in densification,said sections being in communication with each other, said atmospherematerial having a melting point greater than the temperature required tooutgas said compact but less than the temperature at which saidsintering aid evaporates rapidly from said compact; and seal means forcontaining a liquid seal against atmosphere for said cavity.
 2. Theapparatus of claim 1 in which said seal means comprises a channel formedthrough said apparatus from the outside to said reservoir section,opening into said reservoir section adjacent the bottom thereof topermit liquefied atmosphere material to flow into said channel to act asseal material for said apparatus.
 3. The apparatus of claim 1 includingat least one spacer inert to said article in one of said sections forseparating a plurality of said articles.
 4. The apparatus for claim 1 inwhich said atmosphere material is of the same composition as saidsintering aid.
 5. The apparatus of claim 1 in which said specimen andreservoir sections are defined in horizontally spaced relation, and saidmeans for defining said sections comprises a wall between said sectionsdefining a passageway thereacross.
 6. The apparatus of claim 5 in whichsaid vessel includes end caps closing said sections and removable forpurposed of loading said sections, and said seal means comprises achannel formed through one of said caps opening into said reservoiradjacent the bottom thereof.
 7. The apparatus of claim 1 in which saidspecimen and reservoir sections are defined as top and bottom sections,respectively, in vertically spaced relation, between a top wall and abottom wall and said means for defining said sections comprises meansfor supporting said compact spaced from the bottom wall of said cavity.8. The apparatus of claim 7 in which said top wall is formed with a capresting on the top edge of said apparatus and which is removable fromsaid apparatus, said top edge being formed with a channel constitutingsaid seal means, said cap being formed with a lip loosely fitting insaid channel.
 9. The apparatus of claim 8 including seal material insaid channel having a melting point greater than the temperaturerequired to outgas said compact but less than the temperature at whichsaid sintering aid evaporates rapidly from said compact.
 10. Apparatusfor densifying at least one compact under changing pressure conditionsin response to a change in temperature applied to said apparatus,comprising:a vessel having a cavity; said cavity comprising a specimensection for containing a compact to be densified and a reservoirsection, vertically beneath said specimen section, containing atmospherematerial to aid in densification, said sections being in communicationwith each other between a top wall and a bottom wall, said reservoirsection being adjacent said bottom wall; means for supporting saidcompact spaced from the bottom wall of said cavity; and seal means forcontaining a liquid seal against atmosphere for said cavity.
 11. Theapparatus of claim 10 including at least one spacer inert to saidarticle in one of said sections for separating a plurality of saidarticles.
 12. The apparatus of claim 10 in which said top wall is formedwith a cap resting on the top edge of said apparatus and which isremovable from said apparatus, said top edge being formed with a channelconstituting said seal means, said cap being formed with a lip looselyfitting in said channel.
 13. Apparatus for densifying at least onecompact under changing pressure conditions in response to a change intemperature applied to said apparatus, comprising:a vessel having acavity; means for defining in said cavity a specimen section forcontaining a compact to be densified and a reservoir section containingatmosphere material to aid in densification, said sections being incommunication with each other; and seal means for containing a liquidseal against atmosphere for said cavity, comprising a channel formedthrough said apparatus from the outside to said reservoir section,opening directly into said reservoir section adjacent the bottom thereofto permit liquified atmosphere material to flow into said channel to actas seal material for said apparatus.
 14. The apparatus of claim 13 inwhich said specimen and reservoir sections are defined in horizontallyspaced relation, and said means for defining said sections comprises awall between said sections defining a passageway thereacross.
 15. Theapparatus of claim 14 in which said vessel includes end caps closingsaid sections and removable for purposes of loading said sections, andsaid seal means comprises a channel formed through one of said capsopening into said reservoir adjacent the bottom thereof.